The present invention relates to highly proton-conductive polymer electrolyte membranes that are suitable for use in polymer electrolyte fuel cells or in dialysis applications such as electrodialysis and diffusion dialysis and which excel in mechanical strength, as well as a process for producing such polymer electrolyte membranes.
Polymer electrolyte fuel cells have high energy density and hence hold promise for use as power supplies to household cogeneration systems, mobile communication devices and electric vehicles or as simplified auxiliary power sources. In polymer electrolyte fuel cells, a gas-diffusing electrode is composited to both sides of the polymer electrolyte membrane in such a way as to provide a substantially monolithic structure. Hence, the polymer electrolyte membrane not only acts as a proton-conducting electrolyte but also has the role of a diaphragm that prevents the fuel hydrogen or methanol from directly mixing with the oxidant oxygen even under pressure.
One of the requirements that should be satisfied by the polymer electrolyte membrane is that more protons flow. To this end, it is important that sulfonic acid groups responsible for proton conduction should be introduced as much as possible into the polymer electrolyte membrane.
The polymer electrolyte membrane which also has the role of a diaphragm must satisfy other requirements including high mechanical strength and good dimensional stability.
To meet these needs of polymer electrolyte membranes, Nafion® has been commonly used and this is the perfluorinated sulfonic acid polymer membrane developed by Du Pont. However, the Nafion® membrane has the problem that as the amount of sulfonic acid groups responsible for proton conduction is increased, the membrane liquefies, so they cannot be introduced beyond a certain limit. In addition, the membrane strength decreases with the increased introduction of the sulfonic acid groups, causing such a problem as the breakage of the membrane during assembly of a cell unit or cell operation. It has thus been a difficult task to achieve high levels for both proton conductivity and membrane strength. What is more, the Nafion® membrane is very expensive and this has been a great obstacle to the effort in commercializing the polymer electrolyte fuel cell.
Under the circumstances, efforts have been made to develop low-cost polymer electrolyte membranes that can be substituted for the Nafion® membrane. The polymer electrolyte membranes under development are generally classified into three types, fully fluorinated, partially fluorinated hydrocarbon-based, and hydrocarbon-based.
The fully fluorinated polymer electrolyte membrane is highly durable but at the same time it is very expensive. In contrast, the hydrocarbon-based polymer electrolyte membrane and the partially fluorinated hydrocarbon-based polymer electrolyte membrane can be synthesized from quite inexpensive starting materials at low cost and, hence, are expected to have a potential to supply inexpensive polymer electrolyte membranes for polymer electrolyte fuel cells.
In particular, aromatic hydrocarbon-based polymer electrolyte membranes are produced by sulfonating films of aromatic hydrocarbon-based polymers which are super-engineering plastics. Alternatively, polymer electrolyte membranes can be produced by a process which comprises synthesizing a polymer electrolyte by polymerization reaction of an aromatic hydrocarbon-based monomer to which sulfonic acid groups have bonded and then forming a membrane of the polymer (see, for example, JP. 2004-288497 A, JP 2004-346163 A, and JP 2006-12791 A). Because of the aromatic structure it possesses, the produced polymer electrolyte membrane is expected to have high enough mechanical strength. However, these aromatic hydrocarbon-based polymer electrolyte membranes have the same difficulty that the Nafion® membrane has: when more of the sulfonic acid groups responsible for proton conduction are incorporated, the membrane becomes more water-soluble and its mechanical strength decreases. This is because the sulfonic acid groups randomly present in the aromatic hydrocarbon-based polymer chains prevent sharp separation between the hydrophobic portion which contributes to the retention of mechanical strength and the electrolyte layer responsible for proton conduction.
In order to solve this problem, the present inventors previously filed Japanese Patent Application 2007-029223, proposing that a crosslinked structure be introduced by preliminary irradiation of an aromatic hydrocarbon-based polymer film. This process does not involve the membrane-forming step and yet a polymer electrolyte membrane can be obtained by direct sulfonation. A further advantage of this process is that crosslinking provides a higher ion-exchange capacity and contributes to allowing the polymer electrolyte membrane to retain high mechanical strength. However, in order to impart an effective crosslinked structure to the aromatic hydrocarbon-based polymer film substrate, radiation must be applied at a high dose (≧1000 kGy); hence, it has been desired to impart high enough crosslinking density by a lower dose of irradiation.
Japanese Patent Application 2006-227935 proposes that a vinyl monomer having functional groups capable of conversion to sulfonic acid groups be graft polymerized to the aromatic hydrocarbon-based polymer film substrate by radiation-induced grafting. However, on account of the conversion to sulfonic acid groups, the aromatic hydrocarbon-based polymer serving as the substrate in the sulfonation step is also sulfonated, causing a considerable drop in the mechanical strength of the membrane. What is more, due to the low graft polymerizability of the aromatic hydrocarbon-based polymer film, it is difficult to obtain high enough ion-exchange capacity.